I have a canadian shell which has a symbol which I cannot identify. The symbol looks like 2 concentric circles which have a 90degrees opening and in the middle of the circles is the letter s. Does anybody know the meaning of this symbol?

The shell case will almost certainly be an 18 Pdr, and two concentric "C"s are the trade mark of the Canadian Cartridge Company of Hamilton, Ontario, a well known WWI manufacturer. I am not sure about the “S” without seeing a picture of the mark.